1. Field of the Invention
The present invention relates generally to rare-earth doped fiber amplifiers, and, more particularly, to automatic gain control of such amplifiers.
2. Description of the Related Art
Optical fiber communication systems have evolved rapidly over the past twenty years, with substantial increases in both bit rate and repeater spacing. Although, to this point, intensity modulation systems have found most widespread use, coherent systems, that is, systems which mix a weak optical signal with a relatively strong local optical oscillator hold forth the promise of substantial improvements in capacity, bit error rate and repeater spacing distances. Coherent systems could closely approach quantum limit performance. Wavelength division multiplexing (WDM) systems and frequency division multiplexing systems are examples of coherent system. Suitable amplifiers will be required for such systems. One candidate is the rare-earth doped fiber amplifier which typically employs Erbium as the doping material and is therefore commonly referred to as an Erbium doped fiber amplifier (EDFA). With the addition of multiplexing capabilities, the amplifier could act as a system node, in addition to acting as an amplifier.
Although suitable in many ways, a conventional EDFA would experience gain transients with the deletion or addition of one or more channels, due for example, to channel failure or bursty traffic. These transients would be reflected in the remaining channels and could cause substantial degradation in the system's bit error rate.